The category of relations is the category $\mathsf{Rel}$ where
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Objects. The objects of $\mathsf{Rel}$ are sets.
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Morphisms. For each $A,B\in \operatorname {\mathrm{Obj}}(\mathsf{Sets})$, we have
\[ \mathsf{Rel}(A,B) \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathrm{Rel}(A,B). \] -
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Identities. For each $A\in \operatorname {\mathrm{Obj}}(\mathsf{Rel})$, the unit map
\[ \mathbb {1}^{\mathsf{Rel}}_{A} \colon \mathrm{pt}\to \mathrm{Rel}(A,A) \]of $\mathsf{Rel}$ at $A$ is defined by
\[ \operatorname {\mathrm{id}}^{\mathsf{Rel}}_{A} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\chi _{A}(-_{1},-_{2}), \]where $\chi _{A}(-_{1},-_{2})$ is the characteristic relation of $A$ of Example 8.2.1.1.3.
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Composition. For each $A,B,C\in \operatorname {\mathrm{Obj}}(\mathsf{Rel})$, the composition map
\[ \circ ^{\mathsf{Rel}}_{A,B,C}\colon \mathrm{Rel}(B,C)\times \mathrm{Rel}(A,B)\to \mathrm{Rel}(A,C) \]of $\mathsf{Rel}$ at $(A,B,C)$ is defined by
\[ S\mathbin {{\circ }^{\mathsf{Rel}}_{A,B,C}}R \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}S\mathbin {\diamond }R \]for each $(S,R)\in \mathrm{Rel}(B,C)\times \mathrm{Rel}(A,B)$, where $S\mathbin {\diamond }R$ is the composition of $S$ and $R$ of Definition 8.1.3.1.1.
